Process for operating soderberg anodes with vertical contact studs



March 6, 1962 M. 6:. SEM

PROCESS FOR OPERATING SODERBERG ANODES WITH VERTICAL CONTACT STUDS Filed March 5, 1959 FIG. 2.

IN V EN TOR. MATHIAS OVROM SEM ATTORNEYS.

United States Patent 3,924,173 Patented Mar. 6, 1962 Elektrokemisk A/S, Oslo, Norway, a corporation of Norway Filed Mar. 5, 1959, Ser. No. 797,387 Claims priority, application Norway Apr. 19, 1958 Claims. (Cl. 204-67) When using Siiderberg anodes in aluminium furnaces it is now customary to arrange the contact studs vertically in the anode. The contact studs pass through a layer of raw electrode paste which may for example be 25 centimeters deep. This paste is molten and rests directly on the baking zone where the transformation to solid electrode takes place at about 420 C. The baking zone is a well defined limitation between baked and unbaked paste which can easily be localized by inserting an iron rod into the electrode from above.

The contact studs are firmly fixed in the baked electrode paste by the baking and thus supply the electric current to the anode. The studs must be twisted loose from the anode and pulled up before they contact the liquid fluoride bath. A new contact rod is then inserted into the same hole, but in a higher position, for example 25-3O cm. higher up in the electrode. The rod will be baked into the anode by means of raw electrode paste which is filled into the bottom of said hole. If thick flowing electrode paste is used the hole left by the stud is usually left open for a while after the stud has been withdrawn, as the thick electrode paste will not flow down by itself and fill the hole. A suitable special electrode paste can then be filled into the hole before inserting the stud into the anode. This paste may for example be introduced in the molten state. This is the method which has previously been customary. It has however been found that a better quality of anode has been attained by employing a thin-flowing electrode paste containing more binding agent that previous types. Usually such paste contains between 28-33% pitch. The paste will at the temperature in question at the top of the anode be so thin-flowing that the paste will automatically flow into the hole when the contact stud is withdrawn from the anode. This is of course a very practical solution. Several serious difficulties are however connected therewith due to the fact that the anode has a tendency to crack in its lower part, especially at the bottom of the hole for the contact rod, during the strains to which the anode is subjected when the contact rod is twisted loose, as a force up to 20 tons must then be used on a single contact rod. Such cracks can be so large that they form slots through the anode. When the liquid electrode paste flows into a hole left by a contact rod which is surrounded by open cracks of this kind a part of the paste will penetrate through the cracks into the fluoride bath whereby an instantaneous development of tar fumes will take place. If the cracks are large such development of fumes will have the character of a small explosion which can throw a part of the liquid fluoride bath out of the furnace. This may lead to burnings and similar accidents.

The usual procedure to avoid such holes right through the anode is to withdraw the rod from the anode when the lower tip of the rod is so high above the bottom surface of the anode that no through cracks are formed. With such early withdrawing of the rod, the paste plugging the hole under the rod will have time to be baked before reaching the bath through possible cracks. In such cases the rod is usually raised when its tip is located about 25 cm. above the bottom of the anode. This procedure is safe enough but it causes a higher voltage drop in the anode. The inventor has found that one can operate with an average voltage drop of about 0.45 volt if the contact rods are allowed to reach the 15 cm. level above the anode bottom while the voltage drop is usually 0.154120 volt higher at the 25 cm. level. One can thus save at least about 0.15 volt if the tips of the contact rods are allowed to reach the 15 cm. level. This means a saving in current of 3% with the same amperage output, or about 500 kw.-h. per ton of Al. An advantage of equal importance is attained in that the contact rods can operate for a longer time in the anode without having to be withdrawn. As the anode consumption is usually about 2 cm. per day a 5 days longer operating time in the anode is attained by letting the tip of the rod go down to 10 cm. above the anode bottom before being withdrawn.

Applicant has found a way to make use of this possibility by shielding the hole left by the rod against the soft electrode paste by means of a shield which is placed in the soft paste around the rod before it is withdrawn. The shield may be of metal, preferably a non-magnetic metal, for example aluminium. It is usually made of two halves which are hinged together and closed around the rod and then pushed through the soft electrode paste until it reaches the baking zone. The two halves of the shield can if desired, be connected with a suitable tong which permits an easy handling of the shield. As soon as the shield is in place the contact rod can be twisted loose and withdrawn, for example by means of a crane. The small quantity of liquid paste which under these circumstances will flow into the hole will instantaneously become baked at the bottom of the hole without penetrating into the fluoride bath. The further procedure can then be to fill the rod hole with a special paste, for example in the form of cold, crushed pieces, or one can let the electrode paste in the top of the electrode flow into the hole at a suitable speed. One may also fill some dry special paste into the lower part of the rod hole and tamp this paste before the soft paste is introduced. Such a shield of non-magnetic metal is removed from the anode as soon as the bottom of the rod hole has been secured against penetration of soft electrode paste.

The invention is schematically illustrated in appended FIGS. I and II which show a hinged shield of above described type. FIG. I shows a vertical section through a rod with shield and FIG. II a corresponding horizontal section. In the figures, 1 is a contact rod penetrating into the baked part of the anode 2. 3 is the soft paste on top of the anode and 4 indicates the boundary zone between the two layers. As mentioned above this zone is called the baking zone. 5 is the shield which is so dimensioned that there will be a clearing 6 between it and the rod. 7 indicates a tongs for handling the shield. The tongs can be separate or built together with the shield.

Instead of making the shield of metal it may be made of other material, for example wood or silicone rubber. One can also cast the shield in two halves from a dry electrode paste which for example contains 14% binding agent. This has the advantage that it will soon be dissolved in the electrode paste and be incorporated in it. It will, however, protect the rod hole against the soft electrode paste until the bottom of the rod hole is sufiiciently secured. The shield can then simply be pushed aside in the paste, thus permitting the soft paste to flow into the rod hole.

One can also employ shields of usual electrode paste. The shields should then be cooled for example to about 0 C. before being inserted. This has the effect that the melting of the shield will require so long time that there will be time to displace the rod before such paste shield has been destroyed.

I claim:

1. In the process of operating Stiderberg anodes with vertical contact studs, the steps incident to the removal and replacement of such a stud, of surrounding a portion of the stud above the anode with a cylindrical sleeve having an inside diameter larger than the stud while the lower portion of the stud is embedded in the baked portion of the anode, forcing such sleeve down through the unbaked portion of the electrode mass until it meets the baking zone of the anode, removing such stud while the sleeve is in place, adding unbaked paste into the hole left by the stud to fill the bottom of the hole, returning a stud to such hole at a higher position than the original stud but with its end still in the baked portion of the anode and permitting unbaked paste to surround such returned stud to seal the same in place.

2. A process as specified in claim 1, characterized by the fact that the sleeve is of non-magnetic metal which is removed from the anode after the bottom of the hole left by the stud has been secured against penetration by soft electrode paste.

3. A process as specified in claim 1, in which the sleeve is formed in two parts.

4. A process as specified in claim 3, in which the sleeve formed of two parts has a handle on each part to facilitate handling.

5. A process as specified in claim 1, in which the sleeve is formed of hardened electrode paste which gradually becomes incorporated in the raw electrode paste on top of the anode.

References Cited in the file of this patent FOREIGN PATENTS 

1. IN THE PROCESS OF OPERATING SODERBERG ANODES WITH VERTICAL CONTACT STUDS, THE STEPS INCIDENT TO THE REMOVAL AND REPLACEMENT OF SUCH A STTUD, OF SURROUNDING A PORTION OF THE STUD ABOVE THE ANODE WITH A CYLINDRICAL SLEEVE HAVING AN INSIDE DIAMETER LARGER THAN THE STUD WHILE THE LOWER PROTION OF THE STUD IS EMBEDDED IN THE BAKED PORTION OF THE ANODE, FORCING SUCH SLEEVE DOWN THROUGH THE UNBAKES PORTION OF THE ELECTRODE MASS UNTIL IT MEETS THE BAKING ZONE OF THE ANODE, REMOVING SUCH STUD WHILE THE SLEEVE IS IN PLACE, ADDING UNBAKED PASTE INTO THE HOLE LEFT BY THE STUD TO FILL THE BOTTOM OF THE HOLE RETURNING A STUD TO SUCH HOLE AT A HIGHER POSITION THAN THE ORIGINAL STUD BUT WITH ITS END STILL IN THE BAKED PORTION OF THE ANODE AND PERMITTING UNBAKED PASTE TO SURROUND SUCH RETURNED STUD TO SEAL THE SAME IN PLACE. 